The term “compliant mechanisms” relates to a family of devices in which integrally formed flexural members provide motion through deflection. Such flexural members may therefore be used to replace conventional multi-part elements such as pin joints. Compliant mechanisms provide several benefits, including backlash-free, wear-free, and friction-free operation. Moreover, compliant mechanisms significantly reduce manufacturing time and cost. Compliant mechanisms can replace many conventional devices to improve functional characteristics and decrease manufacturing costs. Assembly may, in some cases, be obviated entirely because compliant structures often consist of a single piece of material.
In microelectromechanical systems (MEMS), compliant technology allows each mechanism of a MEMS system to be an integrally formed, single piece mechanism. Because MEMS devices are typically made by a layering and etching process, elements in different layers must normally be etched and formed separately from each other. Additionally, elements with complex shapes, such as pin joints, require multiple steps and layers to create the pin, the head, the pin-mounting joint, and the gap between the pin and the surrounding ring used to form the joint. While pin joints do have difficulties in manufacturing, these complex shapes do have advantages of allowing large displacements and low stresses compared to fully compliant mechanisms.
An integrally formed compliant mechanism, on the other hand, may be constructed as a single piece, and may even be constructed in unitary fashion with other elements of the micromechanism. Substantially all elements of many compliant devices may be made from a single layer. Reducing the number of layers, in many cases, simplifies the manufacturing and design of MEMS devices. Compliant technology also has unique advantages in MEMS applications because compliant mechanisms can be manufactured unitarily, i.e., from a single continuous piece of material, using masking and etching procedures similar to those used to form semiconductors.
In MEMS as well as in other applications, there exists a large need for “bi-stable devices,” or devices that can be selectively disposed in either of two different, stable configurations. Bistable devices can be used in a number of different mechanisms, including switches, valves, clasps, and closures. Switches, for example, often have two separate states: on and off. However, most conventional switches are constructed of rigid elements that are connected by hinges, and therefore do not obtain the benefits of compliant technology. Compliant bi-stable mechanisms have particular utility in a MEMS environment, in which electrical and/or mechanical switching at a microscopic level is desirable, and in which conventional methods used to assemble rigid body structures are ineffective.
Bistable mechanisms present a unique challenge because the compliant elements must be properly balanced so that two fully stable positions exist. Even if a bi-stable design is obtained by fortunate guesswork or extensive testing, conventional optimization techniques are often ineffective because the design space is so complex, i.e., highly nonlinear and discontinuous, with such a small feasible space that gradient-driven methods are unable to reach a workable solution. The likelihood that a stochastic method will stumble onto a solution is extremely small in fully compliant designs. Hence, it is difficult to enhance the fully compliant bi-stable designs, except through additional experimentation.
However, implementation of designs that allow for large displacements of bi-stable mechanisms can provide for mechanisms that are more predictable and require less experimentation to obtain two stable configurations. Adding pin joints to compliant mechanisms can allow for these large displacements to enable bistability without undue experimentation and analysis. Unfortunately, previous MEMS bi-stable designs have encountered difficulties with applying pin joints to non-stationary members. Additionally, various attempts of using non-stationary pin joints have encountered motion problems as the result of stiction, the bonding of moveable members to the microchip substrate.
Further, there is a need for accurate, low power mechanisms for the out-of-plane positioning of microelectromechanical system (MEMS). Such mechanisms are useful in mirror arrays and in erectable structures. One possible means of achieving these accurate, low power mechanisms is to develop out-of-plane bi-stable mechanisms. Several different design concepts for bi-stable mechanisms have been identified including mechanisms composed of rigid and compliant links, bucking structures, and braking or latching devices. Buckling and latching devices have also been used to position out-of-plane mechanisms.
However, out-of-plane compliant bi-stable mechanisms are somewhat challenging because the devices are fabricated in-plane and the elasticity of the compliant segments tends to cause them to return to the plane of fabrication.